The invention relates to optical glass, in particular optical glass for fabricating optical fiber. More especially, the invention relates to Ga:La:S (GLS) glass and related compounds, and to optical fiber and optical devices using such glass.
The production of Ga:La:S (GLS) based fiber without devitrification is an issue that needs addressing. Reaching fiber-drawing viscosities between 104-106 poise at temperatures much less than the onset of crystallization has proven difficult. The high melting temperature (1150xc2x0 C.) required for the processing of Ga:La:S glass restricts the choices of additives that can be used to act as modifiers.
Advantages of Ga:La:S based glass over other competing glass for active and infrared applications are evident through its low-phonon energy, high rare-earth solubility, high glass transition temperature and non-toxicity. However, the tendency to devitrify during fiber drawing hinders the production of small core fiber. Improving thermal properties of Ga:La:S glasses through addition of selected compounds may hold the key to achieving practical fiber.
The addition of a modifier into the Ga:La:S glass matrix provides a way to achieve the improvements required. Modifiers can introduce higher thermal stability for potential fiber drawing and the ability to transmit further into the visible spectrum. One important goal is to provide a GLS glass capable of shifting the Pr3+ emission peak at 1.3 xcexcm closer to the all important 2nd telecommunications window.
For active applications such as fiber amplifiers, these compositional modifications are also beneficial through the reduction of oxide in the glass. Most importantly, it is important that modified Ga:La:S glasses still retain the key characteristics of Ga:La:S.
Some Ga:La:S modifiers that have been studied are as follows [4]:
Further work by J. Wang et. al., [11] has studied the effect of adding Cesium Chloride (CsCl) as a modifier to GLS. GLS:CsCl glasses were characterized to show low-phonon energy and high rare-earth solubility while providing improved thermal and optical properties over GLS. Furthermore, GLS:CsCl glasses exhibited blue shifting into the visible, advantageous for active applications. GLS was successfully doped with up to 30 mol % of CsCl. However, 25 mol % was found to be the optimum as the fiber-drawing capabilities were improved considerably. Initial fiber attenuation measurements revealed losses of 10 dB/m at 1.3 xcexcm. In fluorescence measurements for the Pr3+ ion, both GLS and GLS:CsCl had peak emission at 1.34 xcexcm. A serious drawback with GLS:CsCl glasses during bulk production (of about 170 g), is the shattering of glass ingots in the carbon boats due to its high expansion coefficient.
Other work by J Wang et al [12] studied the effects of adding various lanthanum compounds to GLS, namely LaF3, LaCl3, LaBr3 and LaI3. The effect of adding the halides was characterized in terms of the stability parameter Tx-Tg obtained from differential thermal analysis (DTA) studies. The authors reported that the addition of increasing amounts of LaF3 causes a constant deteriorating effect in the thermal stability of the glasses, whereas the addition of LaCl3 or LaBr3 causes an improving effect on the thermal stability up to approximately 8 mol %, with a peak at around 2 to 3 mol %. The addition of LaI3 into the GLS initially causes a deteriorating effect on thermal stability up to 8 mol %, then it starts to show an improving effect on thermal stability with a peak around 20 mol %. In summary, this work showed that addition of large amounts of LaI3 or small amounts of LaCl3 or LaBr3 may be beneficial to GLS, whereas addition of LaF3 to GLS is harmful.
There is still a need for discovering new GLS compositions that provide some improved properties, while retaining the key characteristics of basic Ga:La:S glass.
The invention relates to a new and improved hybrid of Ga:La:S (GLS) glass, namely a glass of the Ga:La:S group, comprising Ga, La, S, O and F (GLSOF), preferably at least 2% F.
The GLSOF glass may be fabricated by mixing the ingredients gallium sulfide and lanthanum oxide, suitable for making Ga:La:S:O (GLSO) glass with at least 2 mol % lanthanum fluoride. Fabrication of the GLSOF glass may thus be viewed as adding lanthanum fluoride to GLSO glass. GLSOF glass according to the invention has been shown to retain the fundamentally important properties of the Ga:La:S system, while introducing improved thermal stability and spectroscopic properties. The improved thermal stability of GLSOF is somewhat surprising in view of earlier work by Wang et al [12] which showed that the addition of lanthanum fluoride to GLS glass caused deterioration in the thermal stability of the glass. A further advantage is that GLSOF glasses are non-toxic allowing fabrication of non-toxic fiber.
Experiments have demonstrated that GLSOF has thermal properties highly suitable for fiber drawing.
In addition, GLSOF can be made with lower levels of oxide incorporation than comparable GLSO, which is useful for fiber amplifier applications. This is because a GLSOF glass of a certain refractive index can be made with a lower oxygen content than a GLSO glass of the same refractive index. Since carrier lifetime scales with the sixth power of refractive index, this effect is highly sensitive.
The GLSOF glass may comprise:
40-9 mol %, 65-75 mol % or 67.5-72.5 mol % Ga2S3;
0-60 mol % La2S3;
1-60 mol % or 2-25 mol % La2O3; and/or
2, 3, 4, 5 or 6 to any of 60, 40, 25 or 20 mol % LaF3.
The glass may further comprise a rare earth dopant, which may be present at a concentration of at least 50 ppm, 100 ppm or 200 ppm for example.
The dopant may advantageously be Pr. A GLSOF host glass has the unpredicted, surprising and important effect of shifting and broadening the Pr3+ emission peak at 1.3 xcexcm to provide a full width half maximum (FWHM) of 30% at 1.3 xcexcm. These properties make Pr3+:GLSOF of great interest for optical amplifiers and other devices and components for operation in the second telecommunications window.
According to a second aspect of the invention, GLSOF glass is incorporated into an optical waveguide, such as an optical fiber or planar waveguide, for example with a clad of Ga:La:S glass and a core of Ga:La:S:O:F glass. There may be provided an optical waveguide comprising a clad of a clad glass comprising gallium, lanthanum and sulfur and a core of core glass comprising gallium, lanthanum, sulfur, oxygen and fluorine. More specifically, a second aspect of the invention is directed to an optical waveguide comprising gallium sulfide and lanthanum oxide, and a core of glass comprising gallium sulfide, lanthanum oxide and lanthanum fluoride.
The core glass may have a higher mol % of gallium sulfide than the clad glass.
The core glass may have a lower mol % of lanthanum oxide than the clad glass.
The core glass may comprise at least 2 mol % lanthanum fluoride. The clad glass may also comprise lanthanum fluoride. The core glass may have a higher (or lower) mol % of lanthanum fluoride than the clad glass.
According to a third aspect of the invention there is provided an optical fiber preform comprising a core and a clad, wherein the clad comprises a clad glass comprising:
(a) gallium sulfide
(b) lanthanum oxide
the core comprises a core glass comprising:
(c) gallium sulfide
(d) lanthanum oxide and
(e) lanthanum fluoride.
The clad glass of the preform may further comprise lanthanum fluoride.
According to a fourth aspect of the invention there is provided a method of fabricating an optical fiber, comprising:
(a) providing a sample of glass comprising gallium sulfide, lanthanum oxide and lanthanum fluoride; and
(b) drawing the sample into an optical fiber.
In one embodiment a rod-in-tube technique is used in which the sample of glass is provided as a solid rod which is arranged in a glass tube prior to drawing.
In another embodiment a disc extrusion process is used in which the sample of glass is provided as a first disc and arranged adjacent a second disc of a further glass, the drawing comprising extrusion of the first and second discs through an extruder. The term disc here is used following common usage in the art. It will however be understood that the physical shape of the glass xe2x80x9cdiscsxe2x80x9d need not conform to any particular shape, it only being significant to arrange the two glasses one above the other adjacent the extruder.
In a specific embodiment, the core is holey, comprising a plurality of holes extending along the optical waveguide. For example, the optical waveguide may be a holey fiber with a holey core and/or cladding. The plurality of holes may have a characteristic period in at least one direction.
In other embodiments, other processes may be used for GLSOF fiber fabrication.
GLSOF can be used for a number of applications. GLSOF fiber may be used in non-linear devices and fiber amplifiers for telecommunications, for example. Devices based on third order optical non-linear processes can be made. GLSOF glasses show large intensity dependence on refractive index without appreciable linear absorption at the optical communications wavelength. This is required for all-optical switching.